Further mechanistic studies on the PdCl,(pyridine)-catalytic system in aqueous solution are described using the tetrasubstituted allylic alcohol, (E)-2-methyl-d,-4-methyl-1,1,1,5,5,5-hexafluoro-3-penten-2-o1, 3a, and the trisubstituted allylic alcohol, (E)-4-Methyl-1,1,1,5,5,5-hexafluoro-3-penten-2-o1, 6, as substrates. At low [Cl-] the PdCli-catalyzed isomerization of 3a, which can only undergo isomerization into its allylic isomer, was previously found to obey the Wacker rate expression: ki
In contrast, the rate expression for isomerization of 3a by [PdCl,(Py)-] at low [Cl-] was found to be: rate, = k,[PdCl,(Py)-][3a]/[Cl-1.
This rate expression is of the same form as that previously found for the isomerization of 3a by PdCli-at high [Cl-]. This result strongly suggests that the hydroxypalladation by PdCl,(Py)-at low [Cl-] is a truns process as opposed to a cis process with PdCIi-. This expectation was confirmed by stereochemical studies with chiral 3a. The stereochemistry of addition for PdCl,(Py)-was identical to that for PdCli-at high [Cl-]. Independent stereochemical studies have shown this addition to be trans. With PdCl,(Py)-there are two possible routes for olefin oxidation. A cis process similar to that found for PdCli-or a trans process analogous to that previously proposed to explain the truns stereochemistry found at high [Cl-]. Stereochemical studies with 6, which can undergo oxidation, showed that both processes are operative with PdCl,(Py)-at [Cl-] = 0.05 M. Thus addition of a pyridine to the coordination sphere of Pd(II) causes a profound change in reactivity.